问题补充：
people have become masters of their country science can really serve the people.
A. It is only then ;that
B. It is that;when
C. It is only when; that
D. It is when;when
答案：
A. It is only then ;that
B. It is that;when
(答案→)C. It is only when; that
D. It is when;when
解析： 此题是一个强调句型，强调的是时间状语从句，强调句的结构为it is/was + 被强调的部分 + that,句意：只有当人们已经了解了他们的国家的时候科学才真正的为人民服务，所以选C。
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